Patients with chronic airway obstruction (so-called COPD—chronic obstructive pulmonary disease) have a persistent overload of their respiratory muscles, especially at the advanced stage of their disease. Due to the obstruction of the airways and the resulting increase in the airway resistance, they have to make a breathing effort that is several times that of healthy subjects. To guarantee sufficient ventilation, COPD patients at the advanced stage are therefore respirated in a pressure-controlled manner (PCV) when respiratory failure develops, in order to guarantee effective and sufficiently rapid filling of the lungs with breathing gas. At the same time, a comparatively long phase of expiration is desirable when respirating a COPD patient, because the patient must expire the volume of breathing gas breathed in essentially without mechanical support after opening an expiration valve. The comparatively long phase of expiration enables the patient to breathe out with the lowest possible effort. A sufficiently long expiration time is, moreover, necessary in COPD patients to avoid dynamic overinflation of the lungs as a consequence of “air trapping” with the development of an intrinsic PEEP (positive end-expiratory pressure) and increasing inspiratory breathing effort.
The physician has to pay attention to two general conditions now. On the one hand, a maximum pulmonary internal pressure reached plung,max must not be exceeded, because lasting damage to the lung tissue could otherwise develop due to the pressure applied. On the other hand, a desired pulmonary target pressure plung,soll inside the lungs should be reached as fast as possible after the beginning of the phase of inspiration of each respiration cycle. If the desired pulmonary target pressure plung,soll is reached rapidly, early development of a sufficiently long end-inspiratory pressure plateau is possible in the range of the pulmonary target pressure plung,soll, and the diffusion processes between the blood and the breathing gas of the alveoli, which underlie the breathing, take place during this pressure plateau. It is also desirable to reach the pulmonary target pressure plung,soll rapidly because the sooner the pulmonary pressure plung,soll (and hence the end-inspiratory plateau pressure) is reached, the sooner can the phase of inspiration be concluded, and, due to this, the sooner the diffusion processes can be or are also concluded. Concluding the phase of inspiration as soon as possible—measured by the overall duration of each breathing cycle—in turn makes possible a comparatively long phase of expiration.
To meet both of the above-mentioned general conditions—avoidance of harmful pulmonary internal pressures and the shortest possible phase of inspiration or the longest possible phase of expiration—during the respiration of COPD patients, the respiration of such patients has so far been started at the beginning of each breathing cycle with a relatively high initial inspiratory pressure paw(t=0) (initial pressure) markedly higher than the pulmonary target pressure plung,soll. With the intent to prevent damage to the lungs due to a rise in the pulmonary internal pressure plung(t) at excessively high pressure values, the inspiration flow is interrupted after the end of a period set on the respirator and the inspiratory pressure paw(t) is thus lowered to the desired pulmonary target pressure plung,soll.
However, this method is based on empirical values of the particular physician as well as on data from the literature and implies drawbacks and also health risks.
One risk lies in the fact that the physician selects the point in time at which he interrupts the inspiration flow to avoid a further increase in the pulmonary internal pressure above plung,soll too late. Thus, there is a risk of damage to the lungs in case of and based on the respiration of the patient.
Another drawback of this procedure is that the physician lowers the initial pressure paw(t=0) too early when he would like the avoid the above-described, excessively late stopping of the flow and thus the lowering of the respiratory pressure from the initial pressure paw(t=0) to the correspondingly lowered inspiratory pressure paw(t) with certainty. A comparatively late plateau may be formed as a result, the duration of the plateau may be too short, and the plateau pressure may be too low for the necessary diffusion processes. Furthermore, the beginning of the phase of expiration may be delayed, and the duration of the phase of expiration will be disadvantageously short.
Another drawback of this type of setting or predetermination of the respiratory pressures is that the general conditions of respiration may vary or change not only interindividually but also intraindividually. Thus, the physician's empirical values on the point in time at which the above-described pressure lowering shall take place do not apply equally to all patients. Moreover, the respiratory conditions applicable to a particular patient may change significantly in the course of the patient's disease and even during the respiration. The latter happens, for example, when the patient changes his position, attempts to actively support the respiration by his own respiratory activity, to counteract it and the like.